In a Rankine cycle power plant, heat supplied to a boiler containing liquid working fluid vaporizes the working fluid at constant temperature to produce vapor that is supplied to one or more turbine stages where expansion takes place producing work. The heat depleted working fluid exhausted from the turbine stages is transferred to a condenser in which heat is extracted from the vapor condensing it to a liquid that is returned to the boiler for repeating the cycle.
When the working fluid is water, or a fluid having an almost symmetrical, bell-shaped temperature/entropy (T-S) diagram, vapor dropping from the saturated vapor state at the boiler temperature to the condenser temperature along a line of substantially constant entropy (which emulates expansion in a turbine) will result in an end state well within the liquid region of the T-S diagram. The expanded fluid will contain liquid droplets which are detrimental to efficient operation of a turbine. In other words, "wet" vapor is less efficient than dry vapor in transferring energy to a turbine stage; and, as a result, the actual thermodynamic efficiency of a power plant will not be as high as its theoretical efficiency which is directly related to the difference in temperature between the boiler and the condenser. Moreover, "wet" vapor is more corrosive to turbine components than "dry" vapor, and is thus undesirable from this standpoint alone.
The conventional solution to the problem of actual efficiency and corrosion is to stage the turbine, and to superheat the vapor so that the temperature drop in each stage is completed in the vapor region. The use of a superheater, however, reduces the actual efficiency from the theoretical efficiency, and produces a more complex and thus more expensive system.
In order to increase system efficiency, and in order to reduce system complexity, a "dry" working fluid can be used. A "dry" working fluid, such as heptane, for example, has an unsymmetrical, rightwardly skewed T-S diagram with the result that expansion of vapor along a line of substantially constant entropy takes place in the superheated region of the diagram. That is to say, the endpoint of the expansion in the turbine is in the superheated region at the pressure of the condenser, but at a temperature higher than the temperature in the condenser. Thus, the energy extracted from the working fluid will be only a portion of the available energy as determined by the temperature difference between the boiler and the condenser.
Conventionally, a regenerator is used to transfer some of the superheat to the liquid working fluid in the boiler before boiling occurs. However, this type of regenerator is inefficient because it involves a vapor/liquid heat exchanger requiring large heat transfer surfaces, thus resulting in a costly and complex system. Furthermore, with many "dry" working fluids, flammability of the vapor is a major problem. For example, heptane, and many other hydrocarbons and their halogenated derivatives, are ideally suited as working fluids in a Rankine cycle power plant because of their thermodynamic properties and their compatability with the metallic components of a power plant. However, their flammability makes their use hazardous.
It is therefore an object of the present invention to provide a new and improved working fluid for a Rankine cycle power plant which permits full expansion from the boiler to the condenser temperature at an endpoint not in the liquid region, and which reduces the fire hazard, and is thus safer, than conventional "dry" working fluids.